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Mössbauer spectroscopy is a technique in which interaction between the electromagnetic moment of the nuclear charge and electromagnetic field produced by the extra-nuclear electrons are studied. This interaction gives splitting/shifting of the nuclear energy levels.

For the most common Mössbauer isotope, 57Fe, the linewidth is 5x10-9ev. Compared to the Mössbauer gamma-ray energy of 14.4keV this gives a resolution of 1 in 1012, or the equivalent of a small speck of dust on the back of an elephantor one sheet of paper in the distance between the Sun and the Earth. This exceptional resolution is necessary to detect the surface magnetism in nanoparticles.

Chemical Isomer Shift (IS) (): Arises out of the interaction between nuclear charge density and the surrounding ‘s’ electron charge cloud. IS can give information about the spin state as well as the co-ordination number.

Quadrupole Splitting (QS) (): Arises due to interaction between the electric quadrupole moment of the nucleus and EFG created by the electrons. QS can give information about the charge symmetry around the nucleus.

Hyperfine field(Hint) It gives the internal magnetic field of a magnetic material

Nanocomposite magnets comprising a transition group element (Fe, Co, Ni, etc.) embedded in a non-magnetic matrix have been drawing increasing attention for their excellent magnetic properties and widespread technological applications.

MgO was chosen as the matrix as it can easily form OH radicals on its surface, which can have immense biological applications.

The microstructural properties of nanocomposites and nanocrystalline materials (NCM) in general largely depend on the atomic structure of the grain boundaries (interfacial regions/interfaces) because a substantial fraction of atoms are located at the grain boundaries.

Mössbauer spectra of the nanocomposite samples were fitted based on the hypothesis that the grain boundaries possess a different atomic arrangement than that of the bulk crystalline iron, giving rise to a distinct sub-spectrum.

Initially coercivity increases with ball milling duration. We see a sharp increase when the sample is milled from 4 hrs to 12 hrs.

High-energy milling introduces defects in the samples. These defects act as pinning centers for the domain walls increasing the rotational barriers.

This increasing trend almost saturates after 24 hrs of milling as the concentration of the defects also tend to saturate.

Elongated particles have an extra component to the demagnetization energy, which is associated with the shape anisotropy of the particles.

We propose that the sharp increase of coercivity observed for the samples ball milled for 36 hrs and more is due to the combined effect of surface and shape anisotropies associated with the geometrical shape transformation.

To conclude, stable Fe-MgO nanocomposites with sizes varying between 17-40 nm have been prepared by ball milling.

Mössbauer spectra of the samples milled for 36 hrs and above showed an additional component other than the crystalline sextet. This extra component was assigned to the grain boundary fraction.

A distribution of hyperfine fields was needed to fit the grain boundary fraction which indicated its disordered amorphous like structure.

DC magnetization measurements show that the coercivity of the nanocomposites tends to rise with milling time with a sharp increase after 36 hrs of milling which is argued to be due to the combined effect of shape and surface anisotropies associated with the shape transformation.

The decrease in the Ms values with increase in milling time was ascribed to the percentage of magnetic dead layer, which increased with increase in milling time.

Nanocrystalline Fe3O4 and -FeOOH in polyvinyl alcohol gel matrix were synthesized via a novel route, without using any cross-linking agent.

A moderately high-pressure environment of an autoclave instead was used for the synthesis.

TEM studies showed that particles are mostly spherical with average size of 10 nm.

Mössbauer spectra of the as prepared gels at different temperatures showed the presence of superparamagnetic particles in them. The gels were found to be magnetically ordered at 20K giving characteristic six-finger patterns.

DC magnetization studies of the gel were carried out and from the saturation magnetization values the weight percentage of magnetite in the gel was determined.

Mössbauer spectra of the as-prepared gels did not show any appreciable absorption, probably because of their low Lamb-Mössbauer factors in the gel state.

On lowering the temperature down to 60K and finally to 20K, Mössbauer spectra were observed.

For room temperature measurements, the samples were dried by keeping them at ambient temperature in a vacuum desiccator for seven days. The samples obtained henceforth showed appreciable absorption at room temperature.